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Old 10-03-2007, 09:10 AM
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GPS vs Radar Gun

I heard about this on the news this morning.
Quote:
Originally Posted by DEREK J. MOORE
THE PRESS DEMOCRAT
Case pits police radar against GPS in teen's car
Parents of Windsor High student first in county to use tracking device data to fight son's speeding ticket
..
It should be interesting to see what happens in court.
Full Story:
http://www1.pressdemocrat.com/articl...08/1033/NEWS01



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Old 10-03-2007, 09:16 AM
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The next law update will be for every car to be fitted with GPS.
If you break the speed limit, you and the sherrif recieve an e mail.
Yours includes the fine and how to pay it.
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Old 10-03-2007, 10:58 AM
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It would be interesting to know what the difference in the cycle rate on the radar gun is compared to the GPS. The higher the cycle rate the more accurate the rating. I would assume that is the argument the police will end up using is that the cycle rate of their radar gun is higher and more accurate than the GPS. The GPS manufacturer says its accurate to within 1mph, but I wonder over what amount of time/distance that is true?

K
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Old 10-03-2007, 12:22 PM
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interesting... i know lakeville highway, it in my neck of the woods, and nobody goes 45 on lakeville!!
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Old 10-03-2007, 01:00 PM
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That GPS stuff is interesting. I was test driving a BMW 745i one day and it told me I was speeding! I was going about 45 on a 40 MPH street down town, wild.

Brian
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Old 10-03-2007, 01:42 PM
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I dont know for a fact, but a gps unit take a sample every four seconds.
You have to be at a steady speed to get a accrate reading.
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Old 10-03-2007, 02:39 PM
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Check the tech specs for your unit - it will list the sample rates. Most Garmin automotive units update once per second. Some of the cheaper units will not update as often. The Garmin Aviation WAAS units update 5 times per second.

I have a Kenwood GPS nav system in my daily driver. It has a system status screen that will show vehicle speed. It stabilizes sudden variations in speed in about 3 seconds.

Last edited by cucumber1949; 10-03-2007 at 05:42 PM.
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Old 10-04-2007, 06:12 AM
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Can't comment on the technology involved but I think the kids dad is living in La La Land if he really believes that the kid couldn't have dropped 17MPH in a 100 feet.

Will be interesting to see how this plays out.
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Old 10-04-2007, 08:43 AM
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I don't think the kid is going to win just because the court will have to set a precedent. I'm not sure what the cycle rate on a radar gun is but for a laser (which is clearly more) is dozens of samples in a second.

If the GPS only takes a sample once every second the readout should still have shown some acceleration past 45mph if we are to believe the kid was speeding. Otherwise he would have had to accelerated up to 62 and back down to 45 within one second. The other problem is that most GPS units say that they're accurate to within X amount of distance. So there can be as much as a 10, 15 or 20 foot error on the position it gives you. Its easy to see how that error can either work into your favor or not in this example. The error on a radar gun or laser is of course much lower as well. Last, the distance for wireless data to travel for a GPS is much further than a radar/laser, and it requires 3 or more satellites to triangulate the position of the car. Most of us who have GPS see that on the screen its not updated like you're watching TV, it moves in chunks every couple seconds.

IMO the kid probably was speeding and found a way to confuse the system or just got lucky with the readout of the GPS. Sounds like he had a history of doing it and from the above it happens often on that road. I'm sure a GPS vs radar/laser speed reading would show some significant differences in certain situations.

I guess we'll see, I think the kid is going to be paying a ticket though!

K
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Old 10-04-2007, 09:37 AM
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This is a quote from the article.

'Malone's GPS tracker, timed to record his location every half-minute, reported he was going 45 mph at 7:43 a.m.'

If this is true then the guy is not going to have a chance in court. Anyone can see how he could be doing 60 then slow to 30 half a minute later and gps will show a speed of 45.
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Old 10-04-2007, 09:41 AM
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This is a quote from the article.

'Malone's GPS tracker, timed to record his location every half-minute, reported he was going 45 mph at 7:43 a.m.'

If this is true (updates on some GPS units can be changed in 'user settings') then the guy is not going to have a chance in court. Anyone can see how he could be doing 60 then slow to 30 half a minute later and gps will show a speed of 45.
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Old 10-04-2007, 12:02 PM
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Yeah I read that too. I think what that is saying is that it makes a record of the location of the car for a log every 30 seconds, not that it receives its satellite update every 30 seconds, but its certainly still unclear and not going to be in his favor-

K
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Old 10-04-2007, 09:38 PM
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Same town as Twisted Mini's, and Windsor is 7 miles away from me. Interesting...
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Old 10-09-2007, 02:00 AM
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The UK is full of speed cameras, but check this out.
It won´t happen a second time, too much income to be had.
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Old 10-10-2007, 01:28 PM
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I've recently used my Garmin GPS to identify the speed at various time in my 34. I need to know since I recently upgraded my bigs and littles, and I didn't want to find a trustworthy cop to check me with his radar. GPS units are neat devices, but I don''t think I would try to use one as an alibi in court.
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